This invention relates to multi-channel frequency division multiplexing (FDM) systems. More particularly, this invention relates to a circuit for generating the frequency shift tones in the frequency shift keying (FSK) data channels used in the FDM system.
FSK data channels transmit data over a communication link as a serial stream of digital data bits where the data bits are referred to as marks (1's) and spaces (0's). Each mark and space is transmitted as a different sinusoidal tone signal. Each FSK data channel will be operating at a baud rate, with each baud of information comprising a integer number of sequential digital data bits. Each data bit will be transmitted during a bit time. In frequency division mutliplexing (FDM) systems, the data channels of FSK information are located at different frequencies within a single bandwidth of one communication link. Usually this communication link is limited to a 3 kilohertz (3 KHz) bandwidth. As a result, the frequency shift (mark-to-space) and vice versa) of the FSK signals for each channel must be a narrow shift in order that the total number of data channels can fit in the total bandwidth available. For a 16-channel FDM system transmitting at a 200 baud rate, a frequency shift of only 170 Hz may be permitted. For a 75 baud rate, only a 60 Hz shift may be permitted.
In the past, it has been a common technique of switching the tone signals for a mark-to-space or space-to-mark transition instantaneously. Unfortunately, this type of frequency shift produces undesirable sideband energy. In addition to an instantaneous frequency shift, other prior-art devices have used such techniques as voltage-controlled oscillators, dual precision reference sources, or a single precision reference source plus programmable dividers. Although the voltage-controlled oscillator approach allows true frequency shift tone generation, it suffers from two distinct disadvantages--lack of frequency accuracy, and the abrupt shift from mark-to-space and vice versa. The latter disadvantage also is a problem present in the precision reference system.
To overcome the abrupt frequency shift problem of generating unwanted sideband energy, prior-art systems, such as that disclosed in U.S. Pat. No. 3,997,855, have attempted to avoid the abrupt change in frequency during a frequency shift by programming a divide-by-N counter to divide a crystal based reference frequency during a frequency shift to generate discrete intermediate frequencies between the mark and space tones. The selection of the intermediate frequencies generated is controlled by the modulation waveform selected. To minimize the amount of sideband energy generated, a sinusoidal modulation wavefore is required.
U.S. Pat. No. 3,997,855 has attempted to achieve a sinusoidal modulation of the frequency shift by the use of a divide-by-N counter, programmed to generate a sequence of division factors N during the first half of each bit time where a frequency shift is to occur. However, for this prior-art tone generator circuit, the resolution in the frequency of the output tone signal generated varies with the value of N. This is a direct result of the fact that the generated frequency is a function of the inverse of N. A further consequence of the use of a divide-by-N counter approach is the inability of the tone generator circuit to precisely produce all of the CCIR and CCITT FSK tone signals from a single crystal reference signal. This inability to exactly produce the required frequencies results in distortion at the receiving end.
For frequency shifts that are large in magnitude, the need to resolve with a high degree of resolution each of the desired intermediate frequencies for a sinusoidal modulation is less critical. With large frequency shifts, the difference between distortion resulting from some inaccuracies in resolution of the intermediate frequencies and an ideal sinusoidal resolution is not as significant as where small frequency shifts are used. That is, errors in the generation of the exact intermediate frequencies for large frequency shifts do not appear as a significant distortion at the receiving end of the data link. However, where frequency shifts are small, the ability to accurately generate the desired frequencies becomes of great importance. This is true because the same resolution errors present in a tone generator circuit such as '885 for small frequency shifts would result in significant distortion errors at the receiving end. In addition, because the resolution in frequency by a divide-by-N counter varies as a function of N, the sideband noise generated during a frequency shift is not symmetrical. Nonsymmetry in the generation of the sideband noise appears as distortion in the received waveforms where filters are used to remove the sideband energy.
Accordingly, it would be advantageous to provide a frequency tone generator circuit that is able to generate, during a frequency shift, each intermediate tone frequency with the same resolution throughout the shift. It would also be advantageous to provide a tone generator circuit that can resolve each intermediate frequency with a degree of resolution that will produce an accurate approximation to a pure sinusoidal modulation of the frequencies where small frequency shifts are present. Further, it would be advantageous to provide a tone generator circuit that produces symmetrical sideband energy to minimize the distortion introduced by the use of a bandpass filter to filter this sideband energy.